The lemon, Citrus limon, is native to India and South and Southeast Asia and grows best in warm climates. Lemons do well in USDA growing zone 9; hardier varieties can be grown in Zone 8 with precautions taken for winter freezing. They grow well in courtyards and when they are planted against the southern wall of homes, where they get maximum sunshine. Lemons are also successfully grown in pots and containers and moved indoors to avoid freezing weather.
Lemons in Containers
If you grow lemon trees in pots or containers you should move them indoors before temperatures fall below 40 degrees F. They like indoor temperatures of 65 to 75 degrees F. Move them back outside when the temperatures are safely above 40 degrees.
Protecting Young Trees
Soil makes good insulation. If a tree is less than 4 years old, bank about 15 inches of clean soil around the trunk. This will absorb the sun and keep the tree about 15 degrees F warmer. Remove mulch covering the soil. You can wrap the trunk with commercial wraps before adding the soil. Bank the trees in the fall before frost hits and remove the soil in the spring after the danger of frost has passed.
Protecting Older Trees
Drape a light sheet over older trees that are still reasonably small. This should not be a plastic sheet, but rather a fabric that can "breathe." There are commercial varieties available for this purpose. If you expect an extreme frost, place several light bulbs or a small heater under the sheet. This will give you protection of about 6 to 8 degrees F. You should not blanket your trees for more than three days at a time.
Protecting Mature Trees
If your trees are so large you can't cover them with fabric, you still have options. Watch weather forecasts. If a freeze is on its way, clear mulch from around your tree soil to help the soil absorb more sun. Irrigate the soil to help it absorb the maximum heat.
Although it seems counterintuitive, you can spray your tree during a freeze. Ice will form on the tree, giving it 1 degree F protection against extreme temperatures.
Plant Cold-Hardy Varieties
If you live in USDA zone 9 or Zone 8 and would like to grow lemons outside, contact your county extension service to see what lemon cultivars will grow best in your area. The Meyer lemon is widely marketed as a cold-hardy lemon, although it is not a true lemon. Its fruit resembles an orange in shape and in the color of its peel and pulp. It is more seedy and less acidic than a true lemon. It is popular in areas that are susceptible to occasional frosts.